Early foal losses in mares
A mare is bred. She conceives. A foal is planned for. Tragically in a substantial number of cases, within 40 days the embryo has succumbed to death.
Defining early embryonic loss (EEL)Though not as acute or seemingly devastating as viral abortion or mare reproductive loss syndrome, "early embryonic death is the biggest cause of reproductive inefficiency in the mare," says Northwest Equine Reproductive Laboratory (NERL) Director Gordon Woods, DVM, Ph.D. According to the NERL Web site, 'although their mothers may have conceived them without trouble, as many as one-third of embryos die in the oviduct by the time they are 4 days old.'Early embryonic loss, or pregnancy loss before day 40 of gestation, accounts for most pregnancy losses in mares.
"This loss represents a considerable economic loss to the equine industry in the form of increased costs associated with additional breeding of mares and/or decreased foal production," says Dirk Vanderwall, DVM, University of Idaho, Northwest Equine Reproductive Laboratory.
Data from several studies, both experimental and from the field, show that between day 10 and 40 of gestation, there is a mean incidence of embryonic loss, as detected by ultrasonography, of 7 percent, with a range of 2.5 to 25 percent. In a study of 2,562 pregnancies in Thoroughbred mares in New Zealand, 76.5 percent of all detected pregnancy losses occurred before day 49.
Factors affecting early pregnancy lossEmbryos lost within 40 days post-ovulation, may be due to the failure of the endocrine system, the embryo, or the uterine environment.
Potential causes include intrinsic, extrinsic and embryonic factors. Intrinsic factors include oviduct, uterus (endometrial disease), endocrine (progesterone, estrogen), maternal age, lactation, and foal-heat breeding. Extrinsic factors include stress, nutrition, season, climate, sire effects, effects of rectal palpation or ultrasonography, and iatrogenic factors.
Embryonic factors include chromosomal anomalies and immunogenetic influences.
Intrinsic factors-oviductal environmentThe environment and function of the oviduct may play an important role in embryonic loss.
In the mare, transport of the embryo through the oviduct (five to six days) to the uterus is delayed compared to other species. The embryo is in the late morula or early blastocyst stage by the time it reaches the uterus. The mare is also unique because of the retention of unfertilized oocytes within the oviduct. Early embryos are often transported past unfertilized oocytes that are retained at the ampullar-isthmic junction of the oviduct in mares.
Failure of this mechanism responsible for the selective passage of embryos through the oviduct is a possible factor for embryonic loss. In a study looking at oviduct transport though, only one case of complete oviduct blockage was noted in 2,622 mares.